docs: remove link to virsh cmd ref & app dev guide

docs: remove link to virsh cmd ref & app dev guide

Both the application developer guide and virsh command
reference are unmaintained for best part of 8 years, and
so horrifically out of date. This does not give a good
impression to people reading the docs. Now that we are
publishing the man pages online, those are a better
doc to read for virsh.  We can also highlight the API
reference instead of the app dev guide.

The virsh command reference & app dev guide will
still exist on the web root, but will not be linked
to.

Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>